Recent advances in nanomaterials: Synthesis, properties, and applications

1. Introduction

Nanomaterials, defined by their structures with dimensions in the range of 1 to 100 nanometers, form the foundation of nanotechnology a rapidly advancing field that has witnessed remarkable growth since the early 2000s [1, 2]. At this scale, materials exhibit distinct properties that differ significantly from their bulk counterparts, primarily due to quantum confinement effects and a substantial increase in surface area relative to volume. These unique phenomena result in altered chemical reactivity, optical behaviour, mechanical strength, electrical conductivity, and magnetic properties, opening new avenues for scientific exploration and technological innovation [3-5].

The unique characteristics of nanomaterials have driven transformative applications across diverse domains. In electronics, nanomaterials have enabled the miniaturization of devices and the development of high-performance sensors and memory storage units [6]. In energy systems, nanostructured materials are integral to improving the efficiency of batteries, supercapacitors, and photovoltaic cells. In medicine, nanoscale carriers have revolutionized drug delivery by enabling targeted therapy with controlled release mechanisms. Environmental science has also benefited through the use of nanomaterials in water purification, pollutant detection, and sustainable energy generation [6, 7].

Despite the progress, the field of nanomaterials remains complex and rapidly evolving. Researchers continually strive to understand and control the synthesis, stability, toxicity, and long-term behavior of these materials. Addressing these challenges is critical for ensuring the safe, sustainable, and economically viable deployment of nanotechnology across industries [8-10]. This review aims to provide a concise yet comprehensive exploration of nanomaterials. It covers their classification based on dimensionality and composition, elaborates on various synthesis techniques, discusses key physical and chemical properties, surveys a range of current and emerging applications, and reflects on future prospects and challenges. By offering a holistic overview, this work intends to serve as a valuable resource for researchers, students, and industry professionals engaged in the exciting and rapidly growing field of nanomaterials.

2. Classification of nanomaterials

Nanomaterials can be broadly categorized based on their shape, composition, and origin.

2.1. Based on dimensions

Nanotechnology revolves around the use and manipulation of nanomaterials, which are materials that possess at least one dimension in the nanoscale range typically less than 100 nanometers (nm). To put this into perspective, a nanometer is one-billionth of a meter (10⁻⁹ m), making these materials much smaller than those observed at the microscale. This reduction in size leads to remarkable differences in the physical and chemical behavior of materials compared to their bulk counterparts [10]. What makes nanomaterials particularly fascinating is their size- and shape-dependent properties. By merely altering their dimensions or morphology at the nanoscale, entirely new characteristics and functionalities can emerge. Depending on their geometrical configuration, nanomaterials can exist in a variety of shapes such as nanoparticles (spherical), nanorods (elongated), nanosheets (flat layers), and others [11-14], shown in Figure 1. These forms can be grouped based on the number of dimensions that fall within the nanoscale, leading to a standard classification system.

2.1.1. Zero-dimensional (0D) nanomaterials: These materials have all three dimensions (length, width, and height) within the nanoscale. Nanoparticles are a typical example, where the particle is essentially a point-like structure.

2.1.2. One-dimensional (1D) nanomaterials: In this class, only one dimension is within the nanoscale range, while the other two are larger. Nanorods, nanotubes, and nanowires fall into this category, offering unique directional properties due to their elongated form.

2.1.3. Two-dimensional (2D) nanomaterials: These materials have two dimensions in the nanoscale, while the third is significantly larger. Examples include nanofilms, nanolayers, and nanocoatings, which are often used for surface modifications and functional coatings.

2.1.4. Three-dimensional (3D) or Bulk nanomaterials: Here, the material extends beyond the nanoscale in all three dimensions. Despite being larger, these structures are composed of nanoscale units or interactions between nanostructures, such as nanocomposites, core-shell structures, or bundles of nanowires and nanotubes.

2.2. Based on composition

Nanomaterials can be categorized into various classes depending on their morphology, particle size, internal structure, and chemical composition. These classifications help tailor their applications across fields such as electronics, medicine, energy, and environmental science. The major types include carbon-based nanomaterials, metal-based nanoparticles, semiconductor nanomaterials, polymer-based nanomaterials, lipid-based nanomaterials, and nanocomposites.

 2.2.1. Carbon-based nanomaterials

These materials are composed primarily of carbon atoms arranged in distinct nanoscale geometries. The most well-known forms include:

Carbon Nanotubes (CNTs): These are cylindrical structures formed by rolling a single layer of graphene into a tube. CNTs are categorized into single-walled (SWCNTs) and multi-walled (MWCNTs) types, shown in Figure 2(a), depending on the number of concentric graphene layers [15-17] Carbon nanotubes: synthesis, properties and engineering applications. They are renowned for their exceptional mechanical strength, thermal conductivity, and electrical properties, often outperforming steel in strength-to-weight ratio. These features make them valuable for reinforcing composite materials and in nanoelectronics.

Fullerenes: These are closed, cage-like molecules composed entirely of carbon, typically consisting of 60 or more atoms (e.g., C₆₀), shown in Figure 2(b). Their structure resembles a soccer ball, with carbon atoms arranged in a combination of pentagons and hexagons [18, 19]. Fullerenes are allotropes of carbon and exhibit high electron affinity, electrical conductivity, and resilience, making them useful in drug delivery systems, photovoltaics, and superconductors.

2.2.2. Metal-based nanomaterials

Metal-based nanomaterials, composed of metal atoms or ions, are widely researched due to their unique physicochemical properties and broad range of applications. These materials are typically synthesized through chemical or photochemical reduction methods, where metal salts containing divalent or trivalent metal ions are reduced using agents such as sodium borohydride, hydrazine, or citrate. Common examples include silver (Ag), gold (Au), and platinum (Pt) nanoparticles, as well as those based on transition metals like copper (Cu) and rare earth elements such as cerium (Ce). For instance, gold nanoparticles synthesized via citrate reduction are frequently used in biomedical imaging and diagnostics due to their biocompatibility and strong surface plasmon resonance effects [21]. Similarly, silver nanoparticles, known for their antimicrobial activity, have been applied in wound dressings and water purification systems [22]. The high surface-area-to-volume ratio of these materials imparts enhanced catalytic performance, increased adsorption capacities, and improved chemical reactivity. These properties are especially valuable in applications such as catalytic converters, environmental remediation, and drug delivery systems [23]. Additionally, doping metal nanoparticles with other elements for example, incorporating palladium into platinum nanoparticles can significantly improve their catalytic efficiency and thermal stability, particularly in fuel cell and hydrogenation reactions [24]. The tunable properties and multifunctionality of metal-based nanomaterials continue to make them a focal point in nanotechnology research.

2.2.3. Semiconductor nanomaterials

Semiconductor nanomaterials, characterized by their tunable electronic band structures at the nanoscale, have emerged as pivotal components in various advanced technological applications. Recent developments have introduced novel materials and composites that enhance the performance and broaden the applicability of these semiconductors.​

Among the notable advancements is the exploration of two-dimensional materials like MoSi₂N₄. This material exhibits exceptional piezoelectricity, high thermal conductivity, and promising photocatalytic properties, making it a strong candidate for applications in optoelectronics and energy conversion systems. First-principles studies have confirmed its stability and potential in these domains [25]. In the realm of hybrid nanocomposites, the integration of graphene with semiconductor materials has led to significant improvements in gas sensing technologies. Graphene-based composites, due to their large surface area and excellent electrical conductivity, have demonstrated enhanced sensitivity and selectivity in detecting various gases. These properties are particularly beneficial for developing efficient gas sensors operating at room temperature [26]. Furthermore, the incorporation of metal halide perovskites with metal oxides has shown promise in enhancing the stability and performance of optoelectronic devices. These composites leverage the superior photoluminescence and charge transport properties of perovskites, combined with the robustness of metal oxides, to create efficient and durable components for applications such as light-emitting diodes and solar cells [27].

These advancements underscore the dynamic nature of semiconductor nanomaterials research, highlighting the continuous efforts to develop materials with superior properties for next-generation technological applications.

2.2.4. Nanocomposites

​Nanocomposites are multiphase materials where at least one phase possesses dimensions in the nanoscale range (less than 100 nm), leading to enhanced properties compared to their individual components. Recent advancements have expanded their applications across various fields, including structural materials, packaging, sensors, energy storage devices, and biomedical systems.​

In structural applications, incorporating functionalized multi-walled carbon nanotubes (MWCNTs) into aluminum matrices has significantly improved mechanical properties and thermal stability, addressing challenges related to homogeneous dispersion. Similarly, the integration of graphene and silica nanoparticles into polypropylene matrices has enhanced mechanical strength and thermal stability, making them suitable for automotive and aerospace components [28]. For packaging applications, nanoclay-reinforced nanocomposites have been extensively studied due to their improved thermal resistance, flame retardancy, stiffness, and strength. These properties are particularly beneficial in food packaging, where barrier properties against gases and moisture are critical [29].

In the realm of sensors, polymeric nanocomposites incorporating carbon nanotubes and metal oxides have demonstrated enhanced electrical conductivity and sensitivity, making them ideal for developing advanced sensing devices. These materials are being explored for applications ranging from environmental monitoring to healthcare diagnostics [30]. Energy storage devices have benefited from nanocomposites through the incorporation of carbon nanotubes, which enhance electronic conductivity and structural integrity in battery electrodes. Such advancements contribute to the development of batteries with higher energy densities and longer lifespans [31]. In biomedical systems, polymer-based nanocomposites, particularly those incorporating magnetic nanoparticles, are actively utilized in diagnostics and cancer treatment applications. These materials offer targeted drug delivery and improved imaging capabilities, enhancing the efficacy of medical treatments [32]. Overall, the unique properties of nanocomposites, such as superior mechanical strength, thermal stability, and tunable electrical or magnetic behaviors, continue to drive their integration into diverse applications, underscoring their significance in advancing technology and industry.

3. Synthesis of nanomaterials

The synthesis of nanomaterials is a cornerstone in nanotechnology research and applications. The choice of synthesis technique influences the morphology, particle size, surface properties, crystallinity, and ultimately, the performance of nanomaterials in various applications, such as energy storage, catalysis, medicine, and electronics. Nanomaterial synthesis generally follows two main approaches: top-down and bottom-up. These methodologies are selected based on the desired structural and chemical properties of the final product.

3.1. Top-down approaches

Top-down approaches begin with bulk materials and systematically reduce their size to the nanoscale through physical or mechanical means. These methods are widely used for producing nanostructures where control over the shape is less critical than mass production or integration with existing microscale systems.

3.1.1. Mechanical Milling

Mechanical milling or High-energy ball milling (HEBM) is a widely utilized top-down technique for producing nanomaterials, wherein bulk materials are subjected to repeated impact and friction within a rotating drum containing steel or ceramic balls. This method effectively reduces particle sizes to the nanoscale, making it cost-effective and scalable for large-scale production.​ Figure 3 illustrates the working principles of different types of ball mills used for mechanical grinding. Figure 3(a) shows the rotation mechanism where grinding media and material collide inside a rotating drum. Figure 3(b) compares three common mill types tumbler, vibratory, and planetary ball mills highlighting their distinct motion patterns used to achieve efficient size reduction [33].

Recent studies have demonstrated the versatility of HEBM in synthesizing various nanomaterials. For instance, a study achieved large-scale production of zinc oxide (ZnO) nanoparticles, reducing particle sizes from 416.60 nm to 33.27 nm through HEBM, highlighting its potential for mass production of nanomaterials. Similarly, nanocrystalline silver powders have been synthesized from micro-sized silver using HEBM, with detailed analyses of particle size and lattice strain conducted via X-ray diffraction techniques [34, 35]. HEBM has also been instrumental in producing nanocomposites. For example, Fe-doped ZnO nanoparticles prepared via HEBM exhibited enhanced sonophotocatalytic degradation efficiency, demonstrating the method's applicability in environmental remediation. Additionally, the synthesis of carbon nanotube-reinforced aluminum composites through HEBM has shown improved mechanical properties, indicating its usefulness in developing advanced structural materials [34, 36]. While HEBM is advantageous for its simplicity and scalability, it is essential to consider potential drawbacks, such as the introduction of defects and contamination during the milling process. Nonetheless, its ability to produce a wide range of nanomaterials, including alloys and composites, underscores its significance in nanotechnology and materials science.​

3.1.2. Lithography

Lithography remains a cornerstone in nanofabrication, enabling the creation of intricate nanoscale patterns essential for various advanced applications. Traditional techniques like photolithography and electron beam lithography (EBL) offer high precision but are often complex and costly, limiting their scalability for bulk nanomaterial production.​ Figure 4 contrasts conventional photolithography and electron beam lithography both highly precise but limited by cost and scalability with modern bulk methods such as nanoimprint and soft lithography. These newer approaches use stamping-based processes that enable high-volume, versatile, and lower-cost pattern fabrication. Overall, the graphic highlights the technological shift toward scalable nanomanufacturing techniques.

Recent advancements have introduced alternative methods that address these limitations. Extreme ultraviolet (EUV) lithography, for instance, utilizes 13.5 nm wavelength light to achieve sub-10 nm resolution, facilitating the production of smaller, more powerful electronic devices. However, EUV systems are expensive and require complex infrastructure, which can be a barrier for widespread adoption [37]. Nanoimprint lithography (NIL) has emerged as a promising, cost-effective alternative. NIL involves mechanically pressing a patterned mold into a resist material to create nanoscale features. This technique offers high resolution and throughput, making it suitable for applications in semiconductors, photovoltaics, and light-emitting devices. For example, NIL has been successfully employed in fabricating 3D carbon nanostructures for carbon nanotube electronic components, biosensors, and tissue scaffolds [38-40].

Furthermore, advancements in NIL have led to the development of roll-to-roll processes, enabling continuous, high-throughput production of nanostructured materials. This scalability positions NIL as a viable option for large-area applications, such as flexible electronics and wearable devices [43]. In summary, while traditional lithography techniques offer unparalleled precision, their complexity and cost hinder large-scale nanomaterial production. Emerging methods like EUV lithography and nanoimprint lithography provide alternative pathways, balancing resolution, cost, and scalability, thereby broadening the scope of nanofabrication applications [37].

3.1.3. Etching

Etching is a fundamental process in nanofabrication, involving the removal of material layers to sculpt desired nanostructures. Recent advancements have enhanced the precision and applicability of both plasma and ion beam etching techniques.​ Figure 5 contrasts plasma etching based on chemical reactions with reactive ionized gases with ion beam etching, which removes material through energetic ion bombardment. Plasma etching offers high selectivity and vertical profiles suited for integrated circuits, while ion beam etching provides highly directional, precise sculpting ideal for nanowire and fine-structure fabrication.

Plasma etching, particularly reactive ion etching (RIE), has seen significant progress in processing silicon-based materials. Innovations have enabled the fabrication of complex structures such as micropillar arrays and nanowires, which are crucial for applications in micro-electro-mechanical systems (MEMS), sensors, and bioelectronics. Additionally, the development of atomic layer etching (ALE) techniques, utilizing sequential exposures to gases like SF₆ and argon plasma, has achieved sub-nanometer precision in etching silicon dioxide, offering greater control over etch rates and uniformity [42, 43]. Ion beam etching (IBE) has also advanced, providing high-resolution patterning capabilities. Recent studies have demonstrated the effective use of IBE in modifying anodic aluminum oxide (AAO) templates, achieving low porosity levels suitable for applications in nanorod fabrication [44]. These advancements in etching technologies underscore the ongoing efforts to achieve higher precision and complexity in nanostructure fabrication, paving the way for innovations in various fields such as electronics, photonics, and biomedical engineering.

3.2. Bottom-up approaches

Bottom-up approaches assemble nanomaterials atom-by-atom or molecule-by-molecule, often providing superior control over structure, composition, and surface properties. These methods are ideal for synthesizing uniform nanoparticles and complex nanostructures.

3.2.1. Sol-Gel method process

The Sol-Gel method is a versatile and widely utilized method for synthesizing oxide-based nanomaterials, involving the transition from a colloidal "sol" to a solid "gel" phase through hydrolysis and condensation reactions of metal alkoxides or salts. Figure 6 illustrates the four key stages of the sol-gel method, beginning with sol formation and progressing through gelation, aging/drying, and final calcination. Together, these steps transform a liquid precursor into a solid nanostructured material.

This technique offers fine control over porosity, homogeneity, and nanostructure morphology, making it particularly effective for fabricating materials such as silica, titania, and alumina. ​Recent advancements have expanded the applications and capabilities of the sol-gel method. For instance, a study demonstrated the synthesis of silicon oxide (SiO₂) nanoparticles via the sol-gel process, revealing their potential in gas sensing applications due to their specific surface properties [45, 46]. Similarly, titanium dioxide (TiO₂) nanomaterials produced through sol-gel techniques have been explored for their photocatalytic properties, which are beneficial in environmental purification systems [47]. Additionally, the sol-gel synthesis of alumina nanoparticles has been investigated for their structural and thermal stability, making them suitable for various industrial applications [48-50]. Moreover, the sol-gel method has been employed to create complex nanostructures, such as mesoporous manganese oxide (Mn₃O₄), copper oxide (CuO), and magnesium hydroxide (Mg(OH)₂), with controlled sizes and shapes, enhancing their applicability in catalysis and energy storage. The adaptability of the sol-gel process also extends to the fabrication of hybrid materials, such as silica-alumina coatings containing cerium oxide nanofibers, which have shown promise in corrosion resistance and protective coatings [50, 51]. These developments underscore the sol-gel process's significance in nanomaterial synthesis, offering a low-cost, energy-efficient, and controllable approach to producing a wide range of functional oxide nanomaterials for diverse applications.

3.2.2. Hydrothermal and Solvothermal Synthesis method

Hydrothermal and solvothermal synthesis methods are pivotal in fabricating nanomaterials with controlled morphologies and properties. These techniques involve chemical reactions in sealed vessels under elevated temperatures and pressures, using water in hydrothermal processes and organic solvents in solvothermal ones (Figure 7). Such controlled environments facilitate the crystallization of materials with uniform particle sizes and specific structures.​

Recent advancements have showcased the versatility of these methods. For instance, the hydrothermal synthesis of high-entropy amorphous metal oxides has been achieved at low temperatures, resulting in materials with cauliflower-type morphology suitable for oxygen evolution reactions in water electrolysis applications. Similarly, solvothermal synthesis has been employed to produce ultra-small copper ferrite (CuFe₂O₄) nanoparticles with controlled diameters and aggregation states, enhancing their catalytic activity for hydrogen peroxide decomposition [52]. Moreover, the hydrothermal method has been utilized to synthesize antimony tungstate (Sb₂WO₆) nanoparticles, which exhibit excellent photocatalytic efficiency in degrading dyes under LED light irradiation. These examples underscore the importance of parameters such as temperature, reaction time, pH, and precursor concentration in tailoring the properties of nanomaterials [53]. In summary, hydrothermal and solvothermal synthesis techniques offer robust platforms for producing a wide range of nanomaterials, including metal oxides, sulfides, and zeolites, with applications spanning energy storage, catalysis, and environmental remediation.

3.2.3. Chemical vapor deposition (CVD)

CVD is a widely utilized technique for producing high-quality thin films and nanomaterials. In this process, volatile precursors are transported in the gas phase to a heated substrate, where they decompose or react to form a solid material layer. Figure 8 shows the steps of CVD, where volatile precursors are introduced into a chamber and transported to a heated substrate. The precursors then decompose or react on the hot surface, forming a solid, high-quality material layer on the substrate.

CVD is particularly effective for synthesizing materials such as carbon nanotubes, graphene, and various semiconductor nanomaterials.​ Recent advancements have expanded the capabilities of CVD. For instance, low-pressure CVD (LPCVD), atmospheric pressure CVD (APCVD), thermal CVD (TCVD), and plasma-enhanced CVD (PECVD) have been developed to produce high-quality and large-scale monolayer graphene, which is crucial for applications in electronics and optoelectronics. Additionally, CVD has been employed to fabricate three-dimensional architectures of two-dimensional materials like graphene and transition metal dichalcogenides (TMDCs), enhancing their performance in energy storage and catalysis applications [54, 55]. Advanced forms of CVD, such as Plasma-Enhanced CVD (PECVD) and Atomic Layer Deposition (ALD), offer further control over film properties. PECVD utilizes plasma to enhance chemical reactions at lower temperatures, making it suitable for depositing materials on temperature-sensitive substrates.

ALD, on the other hand, allows for atomic-scale precision in film thickness and composition, which is essential for applications requiring ultra-thin and conformal coatings. Recent developments in atmospheric-pressure spatial ALD have achieved deposition rates up to 1.5 nm/min, significantly faster than traditional temporal ALD processes, thereby expanding the potential for high-throughput manufacturing [56]. These advancements in CVD and its variants underscore the technique's versatility and importance in the fabrication of nanomaterials for a wide range of applications, including electronics, energy storage, and catalysis.

3.2.4. Green synthesis ​methods

Green synthesis methods utilize biological entities such as plant extracts, bacteria, fungi, and algae to reduce metal ions into nanoparticles, offering an eco-friendly alternative to conventional chemical and physical synthesis techniques (Figure 9).

These biogenic approaches are gaining attention due to their low toxicity, minimal environmental impact, and biocompatibility, making them particularly suitable for biomedical and environmental applications [57]. Recent studies have demonstrated the successful synthesis of various nanoparticles using green methods. For instance, silver nanoparticles have been synthesized using plant extracts, with the size and shape of the nanoparticles being influenced by parameters such as pH, temperature, and extract concentration. Similarly, the green synthesis of platinum nanoparticles using plant extracts has been shown to produce nanoparticles with controlled size and morphology, which are crucial for their catalytic and biomedical applications [58]. The control over nanoparticle characteristics through green synthesis is further enhanced by understanding the role of various factors. For example, the pH of the reaction medium significantly affects the size and stability of the synthesized nanoparticles. A study highlighted that precise pH control is essential for obtaining silver nanoparticles with desirable characteristics, as pH influences the reduction rate of metal ions and the stabilization of the formed nanoparticles [59]. Moreover, the use of different biological sources, such as various plant parts or microbial strains, can lead to the synthesis of nanoparticles with distinct properties. This diversity in biological reducing agents allows for the tailoring of nanoparticle features to suit specific applications, ranging from drug delivery systems to environmental remediation technologies. ​Figure 10 illustrates the synthesis of nanomaterials through Top-Down and Bottom-Up approaches: